The present invention relates to the field of semiconductor structures, and particularly to electromigration immune through-substrate vias and methods of manufacturing the same.
A metal structure includes a lattice of metal ions and non-localized free electrons. The metal ions are formed from metal atoms that donate some of their electrons to a common conduction band of the lattice, and the non-localized free electrons move with relatively small resistance within the lattice under an electric field. Normal metal lines, excluding superconducting materials at or below a superconducting temperature, have finite conductivity, which is caused by interaction of electrons with crystalline imperfections and phonons which are thermally induced lattice vibrations.
When electrical current flows in the metal line, the metal ions are subjected to an electrostatic force due to the charge of the metal ion and the electric field to which the metal ion is exposed to. Further, as electrons scatter off the lattice during conduction of electrical current, the electrons transfer momentum to the metal ions in the lattice of the conductor material. The direction of the electrostatic force is in the direction of the electric field, i.e., in the direction of the current, and the direction of the force due to the momentum transfer of the electrons is in the direction of the flow of the electrons, i.e., in the opposite direction of the current. However, the force due to the momentum transfer of the electrons is generally greater than the electrostatic force. Thus, metal ions are subjected to a net force in the opposite direction of the current, or in the direction of the flow of the electrons.
High defect density, i.e., smaller grain size of the metal, or high temperature typically increases electron scattering. The amount of momentum transfer from the electrons to the conductor material increases with electron scattering. Such momentum transfer, if performed sufficiently cumulatively, may cause the metal ions to dislodge from the lattice and move physically. The mass transport caused by the electrical current, or the movement of the conductive material due to electrical current, is termed electromigration in the art.
In applications where high direct current densities are used, such as in metal interconnects of semiconductor devices, electromigration causes a void in a metal line or in a metal via. Such a void results in a locally increased resistance in the metal interconnect, or even an outright circuit “open.” In this case, the metal line or the metal via no longer provides a conductive path in the metal interconnect. Formation of voids in the metal line or the metal via can thus result in a product failure in semiconductor devices. Further, accumulation of electromigrated materials as extrusions or hillocks outside the volume of the original metal structures can result in electrical “shorts” with adjacent metal structures.
Electromigration is a function of current density and temperature, and accelerates at high current densities and high temperatures. In addition, electromigration is a function of the grain size and the geometry of the metal line. Specifically, the width of the metal line relative to the grain size can have a significant effect on electromigration. If the width of the metal line becomes smaller than the grain size itself, all grain boundaries are perpendicular to the current flow. Such a structure is also known as a “bamboo structure.” Formation of a bamboo structure results in a longer path for mass transport, thereby reducing the atomic flux and electromigration failure rate.
Further, the length of the metal line can have a significant effect on electromigration. If the length of the metal line is less than a critical length known as the “Blech length,” the metal line is immune to electromigration because the electromigration force is balanced by a stress-induced back-flow of atoms.
In recent years, “three dimensional silicon” (3DSi) structures have been proposed to enable joining of multiple silicon chips and/or wafers that are mounted on a package or a system board. The 3DSi structures increase the density of active circuits that are integrated in a given space. Such 3DSi structures employ through-substrate vias (TSVs) to provide electrical connection among the multiple silicon chips and/or wafers. The length of the TSVs is substantially equal to the thickness of each silicon chip or wafer. Because the thickness of silicon chips and wafers are on the order of 100 microns, the length of the TSVs exceeds the Blech length. See I. A. Blech, J. Appl. Phys. 47, 1203 (1976). Thus, the TSVs as known in the prior art are inherently subject to electromigration, and can fail during operation of the semiconductor chips.